1. Field of the Invention
The field of the invention relates to superconducting digital integrated circuits and, more particularly, to a digital phase rotator which permits phase shifts in a positive and negative direction.
2. Description of the Prior Art
High-speed digital circuits are generally synchronous, with a periodic clock signal distributed throughout the circuit. In some systems, a multi-phase clock signal may be used, with two or more clock signals distributed in parallel. A two-phase clock may have one clock signal delayed one-half a clock period with respect to the second clock signal. Furthermore, it may be desirable in some cases to adjust the phase delay of one or more clock signals, relative to a reference clock. For example, one may want to synchronize an internal clock to that in an external signal, for a clock recovery circuit. In this case, one would want a highly precise adjustment, but this adjustment would likely be much slower than the clock frequency. A circuit optimized for this purpose is often called a phase-delay generator or simply a phase generator.
Alternatively, one might want to design a broadband phase modulator, where the clock phase needs to be adjusted quite rapidly. This type of circuit might be called a phase rotator (or sometimes a phase interpolator). A generic phase rotator or phase generator can be represented by the same block diagram of FIG. 1. This shows a master clock input, one or more phase-shifted clock outputs, and a control input. The control input may in general be an analog signal, but the focus of the invention described herein is on a digital control input signal. Both phase generators and phase rotators have been developed in standard semiconductor circuit technologies, and are examples of the prior art.
It is understood within this application and to one skilled in the art, that the term “clock” or “clock signal” refers to a generally periodic sequence of pulses. Such a pulse train may have a broader range of applications than strictly serving as a clock in a synchronized circuit. For example, a pulse train can serve as a carrier in a digital modulation scheme. Within this application, “clock” does not restrict possible applications of a phase rotator or related circuits.
Ultrafast superconducting digital circuits are based on Josephson junctions integrated together according to RSFQ Logic (Rapid-single-flux-quantum), first proposed by K. Likharev and V. Semenov (1991). Multi-GHz clock rates are routine in this technology, with rates of 100 GHz or higher possible. These very high speeds enable precision timing circuits. Earlier fast timing circuits include a time-to-digital converter (A. Kirichenko, et al., “RSFQ Time Digitizing System”, IEEE Trans. Appl. Supercond., vol. 1, 2001, pp. 978-981.), and a digital programmable phase generator (A. Kirichenko, S. Sarwana, D. Gupta and D. Yohannes, “Superconductor Digital Receiver Components”, IEEE Trans. Appl. Supercond., Vol. 15, No. 2, June 2005, pp. 249-254.). One key circuit element of these devices is an RSFQ toggle-flip-flop or TFF, which can direct input pulses alternately to one or the other of two complementary outputs. TFFs are primary elements of binary frequency dividers and binary counters. Superconducting TFFs have been measured to operate at frequencies in excess of 700 GHz.
The digital programmable phase generator shown in application Ser. No. 11/243,020 is shown in FIG. 2. This follows the model of FIG. 1, with a high-frequency clock input, a decimated clock output, and a set of digital input bits that determine the delay of the output. This is comprised of an alternating chain of m TFFs and m−1 inverters, with the decimated clock a factor of 2m slower than that of the master clock on the left. Further, if the m−1 control inputs represent the bits of a binary number N (the most significant bit is on the right), then the phase delay at the output is given by 360°×N/2m.
RSFQ circuits also are high-precision data converters, based on the fact that a voltage-biased Josephson junction generates a series of single-flux-quantum (SFQ) voltage pulses, each with integrated voltage equal to the flux quantum Φ0=h/2e=2.07 mV-ps. This quantum linearity has enabled superconducting analog-to-digital converters (ADCs) that can directly digitize radio-frequency signals as shown in (O. Mukhanov, et al., “Superconducting Analog-to-Digital Converters”, Proceedings of the IEEE, vol. 92, 2004, pp. 1564-1584). One type of superconducting ADC is known as the phase-modulation ADC, in which the analog input signal is encoded in the modulated timing of a fast SFQ pulse train. Similarly, direct digital synthesis of RF signals (essentially an RF digital-to-analog converter or DAC) can be obtained by precise digital generation of properly timed SFQ pulse trains. For this purpose, a fast superconducting digital phase modulator is required.
The prior art lacked a superconducting phase rotator, which allows phase to be adjusted in both positive and a negative direction. Further, this needs to be done at an ultra-fast rate utilizing RSFQ logic.